Organic semiconductor polymers have been a subject of active research in the field of organic electronics in recent years. For example, the polymers are used in organic electroluminescent elements that emit light when electricity is applied to, organic photoelectric conversion elements that generate power when irradiated with light, organic thin film transistor elements that control the amount of current or the amount of voltage. In such an element, as is the case with inorganic semiconductor material, use is made of an organic semiconductor material obtained by combining a p-type conductive semiconductor material, which is an electron donating material, and an n-type conductive semiconductor material, which is an electron accepting material.
In recent years, since fossil energy of petroleum and the like emit carbon dioxide to the atmosphere, there is an increasing demand of solar cells for the purpose of global environment preservation with the suppression of global warming. Known examples of organic solar cells that use organic photoelectric conversion elements include a wet type dye-sensitized solar cell (Grätzel cell) and a total solid type organic photovoltaic cell. Since the latter does not use any electrolyte liquid, there is no need to take into account evaporation of this electrolyte liquid or liquid leakage, the solar cell can be made flexible, and the structure of the solar cell or production thereof is more convenient than that of the former.
However, photoelectric conversion efficiency and durability of the organic photovoltaic cell are still insufficient. The photoelectric conversion efficiency is calculated according to an expression: short circuit current density (Jsc)×open circuit voltage (Voc)×fill factor (FF). The short circuit current density is improved by using an organic semiconductor material (for example, a donor-acceptor type thiophene derivative copolymer), which has absorption in a wide range from visible light to near-infrared light and which has high carrier mobility. The open circuit voltage is reputedly related to a difference between a HOMO level of the p-type conductive semiconductor material and a LUMO level of the n-type conductive semiconductor material, and if the difference is increased, the open circuit voltage is improved. More specifically, development of a p-type polymer having a deep HOMO and a narrow band gap has been desired, in order to achieve high photoelectric conversion efficiency.
Moreover, controlling of phase separation structure between a p-type organic semiconductor and an n-type organic semiconductor is also important, in order to enhance the photoelectric conversion efficiency. The current mainstream is bulk-heterostructure formed by applying a mixed solution of a p-type organic semiconductor and a n-type organic semiconductor, to allow to cause microphase separation comprising an electron donating phase and an electron accepting phase, due to self-organization. In this structure, the contact area of the interface between the p-type organic semiconductor and the n-type organic semiconductor becomes large, to give efficient charge separation. However, the p-type organic semiconductor and the n-type organic semiconductor are not linked by a chemical bond, and therefore there is a problem of stability of phase separation structure, or durability (thermal durability). In order to stabilize the phase separation structure, proposals have been made on a method of crosslinking a p-type organic semiconductor polymer having a polymerizable group, by light or heat (see Patent Literature 1), or formation of a block polymer of a p-type organic semiconductor and an n-type organic semiconductor (see Patent Literature 2). However, these examples employed a homopolymer, such as poly(alkylthiophene) (PAT) and poly(phenylenevinylene) (PPV), as the p-type organic semiconductor, and therefore absorption is in a shorter wavelength range and the photoelectric conversion efficiency is low. More specifically, there are demands for development of an organic semiconductor which has absorption in a longer wavelength range and which has high durability.